1. Field of the Invention
The present invention relates to toner for developing the electrostatic images of electrophotography, electrostatic recording, electrostatic printing, and the like, developer, toner container, process cartridge, image forming apparatus and image forming method using the toner.
2. Description of the Related Art
In an electrophotographic apparatus or electrostatic recording apparatus, a latent electrostatic image is formed on a photoconductor, to which toner is attached. The toner is transferred to a transfer material, and then fixed to the transfer material by heat to form a toner image. A full-color image formation, a reproduction of colors, is generally done by using toners of four different colors consisting of black, yellow, magenta, and cyan. Development is carried out for each color, and the toner image made up of each toner layer overlaid on the support material is then heated and fixed simultaneously to obtain a full-color image.
In general, for a user who is accustomed to commercial prints, images created by full-color copiers are still not at a satisfactory level, and demands are high for further improving the quality to achieve the fineness and resolution that are comparable to those of photographic and offset prints. It is known that in order to improve the quality of an electrophotographic image, the diameters of toner particles should be small and the distribution of particle diameter should be narrow.
A latent image, either electric or magnetic, is made visible by toner. Toners used for developing an electrostatic image generally include colored particles comprising a colorant, a charge controlling agent, and other additives in a binder resin. Processes for manufacturing toner can be categorized broadly into pulverization and polymerization. Pulverization is a process in which a colorant, a charge controlling agent, an offset preventing agent, and the like are melted, mixed, and evenly dispersed in a thermoplastic resin, after which an obtained toner composition is crushed into small particles and classified to obtain a toner.
With pulverization, toners having somewhat favorable properties can be manufactured, but materials that can be used for toners are limited. For instance, a composition made by melting and mixing the components must be crushed and classified using an apparatus that is economically affordable. For this requirement, the composition should be sufficiently brittle. Therefore, when the composition is actually crushed into particles, the distribution of particle diameters tends to be wide spread. The drawback is that the yield is extremely low when one tries to obtain a reproduced image having favorable tone and resolution because a portion of the toner particles, for example, minute particles of 5 μm or less in diameter and large grains of 20 μm or more, must be removed by classification. In addition, it is difficult in pulverization to evenly disperse a colorant, a charge controlling agent, and the like within a thermoplastic resin. Uneven dispersion of the agents and additives adversely affect the flowability, developability, durability, image quality, and the like of toners.
To overcome such problems in pulverization, toner particles are recently made by other processes such as suspension polymerization (Japanese Patent Application Laid-Open (JP-A) No. 09-43909). However, toner particles manufactured by suspension polymerization have a drawback of poor cleaning ability although they are spherical. For development and transfer of low toner coverage image, there is little residual toner that is not transferred and therefore there is no concern of insufficient cleaning of toner. However, when the toner coverage of an image is high, e.g. a photographic image, a paper jam or the like may result in building up of non-transferred residual toner on a photoconductor on which toner is forming an image but not transferred. Accumulation of such residual toner leads to background smear. Moreover, residual toner contaminates components such as a charging roller, which charges a photoconductor by contact charging, and subsequently reduces the charging performance of the charging roller. Furthermore, concerns for toner particles formed by suspension polymerization include unsatisfactory fixing property at low temperatures and a large amount of energy required for fixing.
On the other hand, another process for manufacturing toner particles is disclosed in Japanese Patent (JP-B) No. 2537503 in which emulsion polymerization is used to form resin fine particles, which are subsequently associated to obtain toner particles having irregular shapes. However, toner particles formed by emulsion polymerization have residual surfactants inside the particles as well as on the surface thereof, even after being washed by water, which reduces the environmental stability of toner charge, increases the distribution of the amount of charge, and causes background smear on a printed image. In addition, the residual surfactant contaminates photoconductor, charging roller, developing roller, and other components causing problems such as insufficient charging performance.
On the other hand, for the fixing process by contact heating, in which heating members such as a heating roller are used, the toner particles must possess releasability, which may be referred to as “offset resistance” hereinafter, from the heating members. In such case, offset resistance can be improved by allowing a releasing agent to exist on the surface of toner particles. In contrast, methods to improve offset resistance are disclosed in JP-A No. 2000-292973 and JP-A No. 2000-292978 in which resin fine particles are not only contained in toner particles, but are concentrated at the surface of the toner particles. However, this approach brings up an issue in which the method increases the lowest possible temperature at which toner is fixed and therefore is unsatisfactory in fixing ability at low temperature, i.e. energy-saving fixing ability.
In addition, this process, in which resin fine particles obtained by emulsion polymerization are associated to provide irregular-shaped toner particles, has another problem. Generally, releasing agent particles are additionally associated to improve the offset resistance. However, the releasing agent particles are captured inside the toner particles and therefore the improvement of the offset resistance is not sufficient. Moreover, since each toner particle is formed by a random adhesion of molten resin fine particles, releasing agent particles, colorant particles, and the like, the composition (the ratio at which each component is contained), molecular mass of the resin, and the like may be different and dispersed for each obtained toner particle. In result, the surface properties of toner particles are different from one another, and it is impossible to form stable images for a long period. Additionally, in a low-temperature fixing system, the resin fine particles that are concentrated at the surface of the toner particles inhibit fixing and therefore the range of fixing temperature is not sufficient.
Recently, a new manufacturing process called emulsion-aggregation (EA) has been suggested (JP-B No. 3141783). In this process, particles are formed from polymers that are dissolved in an organic solvent or the like whereas in suspension polymerization, particles are formed from monomers, and it is said to be advantageous in that, for example, there is a larger selection of resins that can be used and polarity can be controlled. Furthermore, it is said to be advantageous in that it is possible to control the structure of toner particles (core/shell structure control). However, the shell structure is a layer consisting only of a resin and the purpose thereof is to lower the exposure of pigment and wax to the surface. The purpose is not to alter the structure in the resin, and the structure is not capable for such purpose, as outlined in “The characteristics of newly developed toner and the vision for the future” by Takao Ishiyama, and two others from The 4th Joint Symposium of The Imaging Society of Japan and The Institute of Electrostatics Japan on Jul. 29, 2002. Therefore, although the toner particle has a shell structure, the surface of the toner particle is a usual resin without any ingenious feature so that when the toner particle is targeted at fixing at a lower temperature, it is not satisfactory from the standpoint of anti-heat preservability and environmental charge stability and this is a concern.
In any of the above-mentioned processes, suspension polymerization, emulsion polymerization, and emulsion aggregation, styrene-acrylic resins are generally used. Polyester resins are difficult to be made into particles, and it is uneasy to control particle diameter, diameter distribution, and particle shape. Also, their fixing ability is limited when the aim is to be fixed at a lower temperature.
On the other hand, it is known that polyester modified by urea bonds is used for anti-heat preservability and low-temperature fixing (JP-A No. 11-133667). However, this has no ingenious feature administered on the surface, and the environmental charge stability is not satisfactory especially when the conditions are harsh.
Much work has been done from various angles of approach in the field of electrophotography to improve quality, and it is being recognized that it is extremely effective to reduce the size and increase the sphericity of the toner particle. However, as the diameter of toner particles becomes smaller, the transferability and fixing ability tend to decrease, and image quality becomes poor. On the other hand, it is known that by making toner particles round, the transferability rises (JP-A No. 09-258474). In such situation, ever-faster image production is desired in the field of color copiers and printers. For a faster printing, the “tandem method” is effective as disclosed, for example, in JP-A No. 05-341617.
The “tandem method” is a method in which images formed by respective image forming units are overlaid and sequentially transferred onto a sheet of paper that is advanced by a transfer belt so that a full-color image is obtained on the sheet. A color image forming apparatus using tandem method is characteristic in that various kinds of paper can be used, the quality of full-color images are high, and full-color images can be formed at high speed. The high-speed output of full-color images is especially characteristic and no other color image reproduction machines have that characteristic.
There are other attempts to increase speed while improving the quality by using round toner particles. For example, since chemical-like round toner particles form compactly developed toner images on the photoconductor, and the transfer pressure at the time of transfer is evenly imposed onto the toner layer, transfer failures such as transfer yield decrease or dropouts of transfer images is less than that of pulverized toner. However, compared to the pulverized toner in the use over time, the flowability improver added to improve transferability and to give flowability to toner becomes rapidly immersed into a toner surface, radically changing the transferability and flowability. Especially when outputting images with small dimension, in other words, images consuming less toner, in succession, the external additives within toner become immersed in the use over time, withering the effect of improving flowability, and therefore resulting in varied transferability and causing problems such as noticeable nonuniformity over the images, etc. in the present circumstances.